Marvin is Martin's rival, and evidently one who does extreme things (such as "snowboarding down K-2.")
His father is pilot, allowing Marvin to travel around the world on a frequent basis.
He seems to be more much mature and smarter than Martin
He does virtually everything that Martin himself can do (better of course), and even dresses in a style similar to him. Marvin's similarity-yet-dissimilarity to Martin may be a nod to Bizarro's resemblance to yet difference from nemesis Superman.
Marvin first appeared in "Attack of the Mothman" where he shows off his skateboarding skills and his very own comic book. His talents make Martin jealous as Diana, Java, and Jenny faun over him.
He reappears in "The Amazon Vapor" where he helps Martin and Diana on a mission. In "The Third Eye", he works for the Center and has acquired his own U-Watch. Unfortunately for him, after the mission he ends up being deported to the Center's South Pole facility and blames his unfortunate circumstances on Martin.
He continues to hold a grudge against Martin throughout "Body-Swappers" until Martin gives him the majority of the credit for getting rid of the parasite and M.O.M. decided to relocate him to Hawaii.
In a subsequent episode, "Rage of the Leprechaun", Marvin contacts Martin and Diana for help after he was being stalked by a leprechaun that wanted to get back the magical four leaf clover that made anyone who held it incredible lucky that Marvin had taken from him. After the incident, Marvin is chewed out by M.O.M. for keeping the clover instead of getting rid of it. What happens to him after that is unknown.